My invention relates to cryogenic refrigeration systems generally referred to as cryostats. In particular, my invention relates to demand-flow cryostats wherein the expansion of refrigerant through a Joule-Thompson orifice is controlled in response to inventory of liquefied refrigerant or temperature maintained within the insulated container of the cryostat.
It has previously been suggested in the art of demand-flow cryostats to employ a sensing element at the cold end of the heat exchanger and disposed so as to sense the presence or absence of liquid nitrogen in the container at such cold end and to use this sensing device to control the size of the Joule-Thompson orifice. Illustrative of such devices are those shown in U.S. Pat. Nos. 3,269,140; 3,413,819 and 3,517,525. These devices, however, have the disadvantage of normally operating in an on-off mode due to the fact that the sensing element is in contact with the reservoir of liquefied refrigerant, and that before the sensor will react, it must be warmed to above the temperature of the liquid, i.e. it must no longer be in contact with a reservoir of liquid refrigerant. Thus, the orifice is substantially closed when the sensing element is in contact with the liquid, and the orifice is essentially completely open when the sensing element is not in contact with liquid refrigerant. It has also been suggested in the art to employ sensing devices which extend for substantially the total length of the cryostat. While this approach tends to offset the on-off mode of a sensing element contacting the liquid, these sensing elements tend to detect not only the temperature at the cold end of the cryostat, but also the ambient temperature at the warm end of the cryostat. These devices, being sensitive to changes in ambient temperature, therefore, tend to operate efficiently only over an extremely narrow range of ambient conditions. Such cryostats are of the type described in U.S. Pat. Nos. 3,320,755 and 3,457,730. The shortcomings of these latter type of cryostats have been offset substantially by the type of demand-flow cryostat described in my U.S. Pat. No. 3,728,868 which employs a fluid filled bulb as a sensing and activating means spaced apart from the Joule-Thompson orifice towards the warm end of the heat exchanger. Although various suggestions have been made in the prior art to overcome many of the problems encountered when attempting to design an efficient demand-flow cryostat, there is still a need in the field for an inexpensive and easily fabricated cryostat which is not subject to the shortcomings of an on-off mode of operation and which can provide adequate movement of a valve member out of the Joule-Thompson orifice a sufficient distance to permit the blowout of frozen crystals of impurities in the refrigerant, such as, for example carbon dioxide or water.